What makes Nikon and Canon such popular cameras with the pros is not just the cameras and the choice of lenses both from the original supplier and from 3rd parties but also all the other support they give and the infrastructure which is available supporting these brands

Rentals, repair, emergency replacements ...

Sony's new mirrorless FF cameras come with limited number of lenses, how many lenses will they have on offer in 2 years time? Will they have caught up in 5 years?

Sony FF mirrorless may make an inroad into the market, but I do not think they will have toppled either Nikon or Canon in 5 years time.

BTW: the developmet curve of camera technology if flattening beween 2003 and 2008 we saw considerable progress, between 2008 and 2013 we saw less. So much so that I still use my Panasonic LX-3 which was released in 2008 and quite frankly, I am not intending to replace it just yet. So I propose, that the curve will continue to fletten.